FLAME ARRESTER BLOCK FOR A PROTECTION DEVICE AGAINST THE PROPAGATION OF FLAMES

FLAME ARRESTER BLOCK FOR A PROTECTION DEVICE AGAINST THE

PROPAGATION OF FLAMES

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DESCRIPTION

Technological background of the invention

Technical Field of the invention

The present invention relates to devices for protection against the propagation of flames in the presence of explosions and / or detonations caused by any source of ignition. These protection devices are also known as flame arresters or flame arrestors. In particular, the present invention relates to a flame arrester block for use in such protection devices.

Brief overview of the state of the art

Flame arresters are devices whose purpose is to extinguish the flame propagation, for example in the presence of deflagration or detonation, generated by any cause of ignition.

Flame arresters are used as protection devices in various application areas, for example in the petrochemical, chemical, pharmaceutical fields, and, more generally, wherever in the presence of potentially explosive atmospheres.

A flame arrester is a device that, while stopping the propagation of flames, allows the passage of fluids (liquid or gaseous).

The operating principle on which the flame arresters are based is the extinction of the combustion ("fire quenching" or "combustion quenching") due to the transfer of heat from the flame to a body of thermally conductive solid material at a lower temperature. This transfer of heat is made more efficient by forming, in the body of solid material, several narrow passages, through which the flame is forced to pass.

One type of flame arrester commonly used is called "Crimped Metal Ribbon" or CMR. A CMR flame arrester includes one or more flame arrester elements, each composed of layers of smooth and corrugated thin metal sheets alternated to each other, for example arranged in a generally coaxial way with a central mandrel to form a multilayer cylindrical body, for example the whole being enclosed within an outer jacket. The spaces created between the corrugations of the corrugated metal sheets and the adjacent smooth metal sheets define a plurality of passages or channels for the fluid. Such passages, for example of approximately triangular section, extend generally parallel to the axis of the cylindrical body (or with a limited angle of inclination relative to the axis of the cylindrical body).

A CMR flame arrester is for example described in WO 94/00197.

A typical process for manufacturing a CMR flame arrester element calls for taking, from respective reels, two smooth metal sheets, creating a ripple on one of the two metal sheets by means of a toothed wheel, and then wrapping like a spiral the two metal sheets on the central mandrel.

Typically, in CMR flame arresters, one or more flame arrester elements are arranged in axial succession, with interposed spacing elements to generate turbulence in order to increase the heat exchange efficiency and thus ensure the stopping of the flame without having to increase the number of flame arrester elements.

Summary of the Invention

The Applicant has observed that in spite of their common use, the CMR flame arresters have some drawbacks.

For example, they can be easily damaged during handling, in particular during maintenance operations. The damage can cause the alteration of the size of the passages for fluids, which may result in the extremely dangerous inability to stop the propagation of the flame, or the misalignment of the metal sheets, which can result in increased pressure drops.

Also, since the size of the passages or channels for fluids are extremely limited, such passages can easily be clogged by deposits, which makes a periodic maintenance of the flame arresters essential.

Moreover, the CMR flame arresters involve a significant pressure drop. The pressure drop depends on the internal construction of the CMR flame arrester, i.e. on the number of flame arrester elements, on the cell size of the elements ("gap" or height of the passages or channels for the fluid), the height of the elements, on the inclination of the passages or channels of the elements and on the thickness of the metal sheets that compose the elements. This pressure drop is a function of the flow rate, the fluid type and its physical condition, and increases greatly when the flame arrester element becomes dirty, with the consequent obstruction of the passages or channels. The dirtying can be very high in the case of fluids containing impurities and sticky fluids.

Another drawback of CMR flame arresters is their relative complexity of construction, given the rather large number of different parts of which they are composed (central mandrel, smooth metal sheets, undulated metal sheets, external jacket), which require specific processes for their manufacture using dedicated automatic or semi-automatic machines that make the assembling rather complex. All this makes CMR flame arresters quite expensive. Another consequence of the complexity of construction of CMR flame arresters is that they, once assembled, cannot be completely disassembled for cleaning, because it would be virtually impossible to reassemble them and ensure the initial security. For their clearing, it is necessary to resort to the use of solvents or high pressure steam (the only way to penetrate into the interstices of the CMR flame arrester), and the operations are therefore complex, lengthy and sometimes not entirely effective.

Furthermore, it is critical to obtain high accuracy and repeatability of the production, and a robustness of the flame arrester element against flexion and deformation. It is not easy to obtain a low ovalization of the flame arrester elements (determined by the spiral winding of the metal sheets on the central mandrel).

The Applicant has faced the problem to solve these and other drawbacks of the prior art.

According to the present invention, there is provided a flame arrester block for use in protection devices against the propagation of flames, whether they are a consequence of explosions or detonations.

The flame arrester block includes:

- at least one first plate having an outer perimeter and at least one opening in a region internal to the outer perimeter, and

- a second, closure plate stacked on said at least one first plate along a stacking direction and spaced apart from the at least one first plate so as to define, between said at least one first plate and said second plate, a gap transverse to said stacking direction.

Preferably, the second plate is spaced apart from the first plate by at least one spacer.

Said at least one first plate may comprise a plurality of first plates stacked one on the other along said stacking direction, and mutually spaced apart by respective spacers to define, between adjacent pairs of first plates of said plurality of first plates, a plurality of interspaces or gaps.

Said first plate may be in the form of a quadrangular frame, particularly, square-shaped or rectangular or rhomboidal, elliptical, star-shaped, quadrangular with lobes or annular-shaped (i.e., having the shape o fan annulus).

Said second plate is preferably of a shape corresponding to the shape of the first plate, in particular the second plate may be disc-shaped or square-shaped.

Said at least one spacer may be a washer.

Said at least one spacer may also be a thickened portion of the material of the first plate and / or the second plate. Said at least one first plate and said second plate are preferably clamped, tightened to form a pack.

According to the present invention, there is also provided a device for protection against fire and / or explosion, comprising at least one flame arrester block according to the above.

The protection device may comprise one among: an end-of-line protection device, an in-line protection device, an overpressure valve (relief valve or vent valve), an underpressure valve or vacuum valve, a breathing valve, for the protection against the spread of flames in the presence of deflagration or detonation.

The protection device may comprise two flame arrester blocks arranged one opposed to the other with facing second, closing plates and contained in a jacket, for allowing a bidirectional flow of fluid and a bidirectional protection against propagation of flames.

The protection device may comprise at least a first and a second flame arrester blocks arranged concentrically.

The protection device may comprise:

- for use with class IIA fluids according to standard ISO 16852: 20 first plates with thickness of 1 mm, external diameter of 15 mm, spaced apart one from the other a distance of 0.5 mm;

- for use with class IIB fluids according to standard ISO 16852: 20 first plates with thickness of 1 mm, external diameter of 20 mm, spaced apart one from the other a distance of 0.3 mm;

- for use with class IIC fluids according to standard ISO 16852: 15 first plates with thickness (s) of 1.5 mm, external diameter of 25mm, spaced apart one from the other a distance of 0.15 mm.

The Applicant has found that such constructions are suitable to make a protection device for "endurance burning" compliancy.

The flame arrester block according to the present invention is simple to manufacture (being composed of few component parts, each of which is of simple construction or even already commercially available), assembly and installation, and therefore is inexpensive.

In addition, the flame arrester according to the present invention is simple to maintain, because it is easy to disassemble and reassemble it, and moreover the cleaning can also be performed with normal abrasive systems and without the need to resort to the use of solvents or high pressure steam. Another significant advantage of the flame arrester block according to the present invention is its modularity: in fact, by increasing the number of stacked (and mutually spaced apart) plates, pressure drops can be reduced substantially at will, without compromising the performance in terms of the ability to stop the propagation of flames.

Brief description of the appended drawings

These and other features and advantages of the present invention will be made apparent by the following detailed description of possible exemplifying and not restrictive embodiments of the present invention, description that will be carried out referring to the accompanying drawings. In the drawings:

- Figure 1 schematizes a possible application of a flame arrester block according to an embodiment of the present invention;

- Figure 2 shows, partly in section, an end-of-line protection device with a flame arrester block according to an embodiment of the present invention;

- Figure 3A shows, in section, a detail of the flame arrester block of Figure 2;

- Figure 3B shows, in section, a detail of an alternative embodiment of the end-of-line protection device of Figure 2, with two nested flame arrester blocks;

- Figures 4 and 5 show, respectively, in plan view from below and in section along the plane indicated by V-V in Figure 4, a part of the flame arrester block of Figure 2;

- Figure 6 shows, enlarged, a detail of Figure 5;

- Figure 7 shows the application of flame arrester blocks according to an embodiment of the present invention on a breathing valve for a tank of, for example, petroleum or its derivatives;

- Figure 8 shows in section an embodiment of an in-line protection device with flame arrester blocks according to an embodiment of the present invention;

- Figure 9 shows in section another embodiment of an in-line protection device with flame arrester blocks according to an embodiment of the present invention, and

- Figure 10 shows in section another embodiment of an in-line protection device with flame arrester blocks according to an embodiment of the present invention.

Detailed description of exemplary embodiments of the invention

With reference to the drawings, Figure 1 shows schematically a possible application of a flame arrester block according to an embodiment of the present invention. Reference 105 indicates a reservoir, for example a tank of a petrochemical plant (or a chemical plant, a pharmaceutical plant, etc.). The tank 105 is intended to contain a flammable liquid 110, for example oil or a liquid derived from petroleum refining, exhaling vapor 115.

The tank 105 is provided with a vent duct 125 for venting of flammable vapors 115 which develop internally. At the end of the vent duct 125 an end-of-line protection device 130 is mounted, provided with a flame arrester block to stop the potential propagation of flames along the vent duct 125 and within the reservoir 105 in the presence of deflagrations that trigger the combustion of the flammable vapors 115, for example explosions triggered by an electrical discharge, for example due to atmospheric events such as lightning 135 from storm clouds. The end-of-line protection device 130 is preferably provided with a rain cover 140.

Figure 2 shows, partly in section, the end-of-line protection device 130 of Figure 1, according to an embodiment of the present invention. In Figure 3A there is shown, in section, a detail of the flam arrester block which equips the end-of-line protection device 130 of Figure 2, while Figures 4, 5 and 6 show the flame arrester block respectively in bottom plan view, in section along the plane indicated V-V in Figure 4, and in a detail.

The end-of-line protection device 130 of the embodiment of Figure 2 comprises a hollow body 205 comprising a first end flange 210 for mounting, for example, to the vent duct 125 shown in Figure 1. The hollow body 205 includes, from the first end flange 210, a first portion 215, for example substantially cylindrical, followed by a second portion 220, for example conical, terminating in a second end flange 225.

On the second end flange 225 there is mounted a flame arrester block 230. In the example considered, above the flame arrester block 230 a rain cover 235 is mounted (however, as explained below, thanks to the structure of the flame arrester block according to the present invention, the provision of rain cover 235 is not essential).

The flame arrester block 230 includes a plurality of plates 305, for example metal sheets, preferably made of carbon steel or stainless steel or special steel, for example having the shape of a circular crown (annulus), stacked along the longitudinal axis X of the end-of-line protection device 130 corresponding to the axis of the inner hole of the hollow body 205. Preferably, the annulus- shaped plates 305 are of equal diameter, both internal and external. For example, the inner diameter of the annulus-shaped plates 305 may correspond to the inner diameter of the second end flange 225. The outer diameter of the annulus-shaped plates 305 may correspond to the outer diameter of the second end flange 225. At the top (according to the orientation of Figure 2), the flame arrester block 230 is closed by a closure plate 310 in the shape of a disc (i.e., a circle) of diameter preferably equal to the outer diameter of the annulus-shaped plates 305. Also the disc-shaped plate 310 may be a metal sheet, preferably made of carbon steel or stainless steel or special steel.

The annulus-shaped plates 305 and the disc-shaped closure plate 310 are clamped, tightened to form a pack, being kept suitably spaced apart by spacers 315, such as washers. For clamping the annulus-shaped plates 305 and the disc-shaped closure plate 310 with interposed the respective spacers 315 in a pack, bolts or tie rods 320, 405 may be used (as in the example shown), inserted in through holes formed in circumferential succession and in corresponding positions: in the vicinity of the perimeter of the disc-shaped closure plate 310, in the vicinity of the outer perimeter of the annulus-shaped plates 305, and advantageously on the second end flange 225.

Above the disc-shaped closure plate 310, the rain cover 235 can be mounted, being a substantially cap-shaped element with a diameter preferably greater than the diameter of the flame arrester block 230, that is, preferably greater than the outer diameter of the annulus-shaped plates 305. For the assembly of the rain cover 235 (when provided) it is advantageous to exploit the same tie-rods 320 already used for clamping into a pack the plates 305, 310 and the spacers 315 that form the flame arrester block 230. As mentioned above, the provision of the rain cover 235 is however not essential, since the closing plate 310 already performs rain cover functions, preventing the penetration of raindrops within the hollow body 205.

The flame arrester block 230 thus formed gives rise to a plurality of radial passages for fluid (vapor or gas), said passages being transverse, for example substantially orthogonal to the X axis, and being defined by the interspaces (gaps), created by the spacers 315, between the various pairs of annulus-shaped plates 305 adjacent to each other and by the interspace between the last annulus-shaped plate 305 of the stack and the disc-shaped closure plate 310.

The thickness of the spacers 315, and then the distance between two adjacent annulus- shaped plates 305, or between the last annulus-shaped plate 305 of the stack and the disc-shaped closure plate 310, are designed such that a possible flame 330 that generates outside the flame arrester block 230 does not propagate inside the hollow body 205 by the action of extinction ("quenching") determined by the transfer of heat from the flame to the plates 305, 310 of the flame arrester block 230.

The constructive parameters that determine the composition of the flame arrester block 230 are few and simple, and are summarized in the following: the thickness s of the annulus-shaped plates 305, the distance d between the adjacent annulus-shaped plates 305 in the stack, and the distance d between the last annulus-shaped plate 305 of the stack and the disc-shaped closure plate 310, the width (indicated with L in Figure 6) of the circular crown formed by each plate 305, the outer diameter D of the annulus-shaped plates 305, the number of annulus-shaped plates 305 stacked to form the pack, the material of the annulus-shaped plates 305.

The choice of the values of these constructive parameters depends on the fluid of the specific application of interest and on the operating conditions of the fluid in the considered application (for example, pressure and temperature inside the tank 105).

In particular, the distance between the adjacent annulus-shaped plates 305 and between the last annulus-shaped plate 305 of the stack and the disc-shaped closure plate 310 may be of the order of magnitude of the value of the parameter known as MESG ("Maximum Experimental Safe Gap") related to the particular fluid of interest for the specific application from time to time considered. The MESG parameter is defined in ISO 16852, which is the reference standard for the test of flame arresters. For example, the reference standard establishes that the value of the MESG for class IIA fluids (e.g. propane) is 0.9 mm, the value of the MESG for class IIB3 fluids (for example ethylene) is of 0.65 mm, and the value of the MESG for class IIC fluids (e.g. hydrogen) is 0.5 mm. Given the specific fluid of the application of interest, the distance between the adjacent annulus-shaped plates 305 and between the last annulus-shaped plate 305 of the stack and the disc-shaped closure plate 310 is advantageously chosen so as to be less than or at most equal to the value of the MESG established by the standard for that fluid.

The number of annulus-shaped plates 305 to be stacked to form the flame arrester block 230 depends instead on the maximum acceptable pressure drop. By increasing the number of plates 305 in the stack the pressure drop decreases.

For example, using annulus-shaped plates 305 having an inner diameter of 200 mm for a class IIA fluid, about 60 plates 305 spaced apart by about 0.9 mm can be provided, if it is desired to have full flow, that is, without reduction of the flow section of the gas. By doubling the number of plates 305, the pressure drop is halved.

Advantageously, even in the presence of persistent flames 330 (so-called "endurance burning"), i.e. flames that are not extinguished after a short time but remain for an indefinite time, it is not necessary to provide systems for opening the rain cover 235 to dissipate the generated heat: as shown schematically in Figure 3A, the flame 330 is not localized at the top of the flame arrester block 230 (which instead happens in the CMR flame arrestors), and then in the area covered by the rain cover 235, what prevents the dissipation of the heat, but all around the flame arrester block. The Applicant has found that suitable constructions of flame arrester blocks 230 for sustaining persistent flames, i.e. tolerating endurance burning, are the following:

- for class IIA fluids: 20 annulus-shaped plates 305 with thickness s of 1 mm and external diameter D of 15 mm, spaced apart one from the other a distance d of 0.5 mm;

- for class IIB fluids: 20 annulus-shaped plates 305 with thickness s of 1 mm and external diameter D of 20 mm, spaced apart one from the other a distance d of 0.3 mm;

- for class IIC fluids: 15 annulus-shaped plates 305 with thickness s of 1.5 mm and external diameter D of 25mm, spaced apart one from the other a distance d of 0.15 mm.

Optionally, one or more CMR flame arrester elements (not shown in the drawings) may be associated with the flame arrester block 230, by placing the CMR flame arrester elements in series with, preferably upstream, the flame arrester block 230, for example inside the hollow body 205, e.g. in a position corresponding to the second end flange 225.

It is also possible to provide two (or, possibly, more) flame arrester blocks 230 arranged concentrically, coaxially (along the X axis), with one flame arrester block 230 of smaller external diameter D arranged in a nested way inside another flame arrester block 230 of greater external diameter D. The two flame arrester blocks 230, preferably not in thermal contact with one another, results in series from the viewpoint of the fluid path. An example of such an arrangement is visible in Figure 3B, wherein the outer flame arrester block is denoted 230-e, whereas the inner flame arrester block is denoted 230-i.

The flame arrester block 230 described above, in addition to being applicable to an end-of- line protection device 130, can be applied to other protection devices against fire and explosion, for example for the protection of tanks 105 intended to contain flammable liquids 110, such as oil or its derivates, and which give off flammable vapors 115.

For example, as shown in Figure 7, one or more flame arrester blocks 230 can be mounted on a breathing valve 705. Breathing valves are known valve protection devices designed to prevent the deformation of the tank caused by an increase or a decrease of the tank internal pressure, for example resulting from the filling or emptying of the tanks with the liquid that they are intended to contain.

With reference to Figure 7, the breathing valve 705 includes a vacuum (underpressure) valve 710 and an overpressure (relief) valve 715 coupled together and in fluid communication. The vacuum valve 710 is activated to compensate (with air from the outside environment) a depression (underpressure) in the tank to which the breathing valve 705 is applied. The overpressure valve 715 is activated to compensate (by means of vent into the atmosphere) an overpressure in the tank to which the breathing valve 705 is applied.

The breathing valve 705 comprises a first valve hollow body 720 for the vacuum valve 710 and a second valve hollow body 725 for the overpressure valve 715, the first and the second valve bodies 720 and 725 being coupled to each other and in fluid communication (in other embodiments, the first and the second valve bodies 720 and 725 may be a single body). The first valve body 720 includes a flange 730 for the attachment to the tank (or to a flanged pipe in fluid communication with the interior of the tank), a first opening 735 to put the interior of the first valve body 720 in communication with the atmosphere, and a second opening 737 for fluid communication with the second valve body 725. In the first valve body 720 a first shutter 740 is housed, to selectively open or close the opening 735, said first shutter 740 operating by simple weight force (or by reaction of a spring). The second valve body 725 includes a first opening 745 for fluid communication with the first valve body 720 and a second opening 750 to put the inside of the second valve body 720 in communication with the atmosphere. Inside the second valve body 725 a second shutter 755 is housed, to selectively open or close the second opening 750, said second shutter 755 operating, as the first shutter 740, by simple weight force (or by reaction of a spring).

Associated with the first opening 735 of the first valve body 720 and the second opening 750 of the second valve body 725 there are a first and a second flame arrester blocks 230-1 and 230-2 of the type described above in relation to Figures 2 - 6.

Above the second flame arrester block 230-2 there is mounted a rain cover 765 (although, as described above, the provision of the rain cover is not essential, since the closure plate 310 of the second flame arrester blocks performs itself the functions of shelter against the penetration of raindrops).

The breathing valve 705 is, thanks to the provision of the two flame arrester blocks 230-1 and 230-2 provided at the openings to the atmosphere of the valve itself, protected against the propagation of flames that develop in the atmosphere.

In addition to an end-of-line protection device and to a breathing valve, such as the end-of- line protection device 130 and the breathing valve 705 described above, the flame arrester block 230 may be associated with other protection devices, such as for example overpressure valves, underpressure valves and in-line protection devices, such as that shown by way of example in Figure 8 and indicated therein with reference 805.

The in-line protection device 805 is used for example in pipes of petrochemical, chemical, pharmaceutical plants, etc., and more generally whenever in the presence of a flammable fluid (vapor or gas), to allow the flow of fluid but preventing the transmission (propagation) of a possible flame from a portion of the pipeline to another portion of the pipeline. The flame may arise for example in the presence of a deflagration or detonation within the pipeline. In particular, the in-line protection device 805 shown in the figure is bidirectional, to prevent the propagation of flames from both sides.

The in-line protection device 805 comprises two junctions having flanged ends 810 and 815 for the attachment to a first and a second portion of a pipeline to be protected (not shown in the figure). In its central part, the in-line protection device 805 has a pair of flame arrester blocks 230-3 and 230-4 of the type described above in relation to Figures 2 - 6 opposed to each other, that is, with the respective closing plates 310- 3 and 310-4 facing one another. The two flame arrester blocks 230-3 and 230-4 are accommodated within a jacket 820 of the in-line protection device 805. The arrows in the figure schematize a possible fluid path (the opposite path is also possible).

Figures 9 and 10 show other possible embodiments of bidirectional in-line protection devices 905 and 1005, respectively.

The in-line protection device 905 comprises two junctions having flanged ends 910 and 915 for the attachment to a first and a second portion of a pipeline to be protected (not shown in the figure). In the central part thereof, the in-line protection device 905 has a pair of flame arrester blocks 230-3 and 230-4 of the type described above in relation to Figures 2 - 6 opposed to each other, having a common closing plate 310, and forming an assembly. The two flame arrester blocks 230-3 and 230-4 are surrounded by a jacket 920 having a lateral (e.g., cylindrical) wall 920-1 and two end walls 920-2, the end walls 920-2 being attached (e.g., soldered) to a respective one of the junctions 910 and 915. The jacket 920 is tightened between a pair of rings 925 by means of tie-rods 930, preferably arranged in circumferential sequence. Screws 935 secure each one of the rings 925 to a respective one of the end walls 920-2 of the jacket 920. This in-line protection device 905 can be dismounted directly in-situ (i.e., without the need of removing the protection device from the pipeline), and also enables the in-situ inspection and cleaning of the flame arrester blocks 230-3, 230-4. The protection device 905 can be opened by loosening the tie-rods 930. In order to gain access to the flame arrester blocks 230-3, 230-4, the screws 935 are unscrewed, so as to make the jacket 920 free to move along the X axis uncovering the flame arrester blocks 230-3, 230-4. The assembly of the two flame arrester blocks 230-3, 230-4 is held within annular grooves formed in the end walls 920-2 of the jacket 920 (with sufficiently low tolerance in order to avoid the propagation of flames) but is free to move along the X axis. The in-line protection device 1005 comprises two junctions having flanged ends 1010 and 1015 for the attachment to a first and a second portion of a pipeline to be protected (not shown in the figure). In the central part thereof, the in-line protection device 1005 has a pair of flame arrester blocks 230-5 and 230-6 of the type described above in relation to Figures 2 - 6, arranged opposed to each other, with facing closing plates 310-5 and 310-6. The two flame arrester blocks 230-5 and 230-6 are surrounded by a jacket 1020 having a lateral (e.g., cylindrical) wall 1020-1 and two end walls 1020-2, and which is tightened between a pair of rings 1025 by means of tie-rods 1030, preferably arranged in circumferential sequence. Each of the two flame arrester blocks 230-5 and 230-6 is mounted to a respective one of the jacket end walls 1020-2 by means of tie-rods 1035. Also this in-line protection device 1005 can be dismounted directly in-situ (i.e., without the need of removing the protection device from the pipeline), by lateral extraction of the jacket 1020 (with the flame arrester blocks 230-5 and 230-6 contained therein) after having loosened the tie-rods 1030. Once the jacket 1020 has been extracted, access to the flame arrester blocks 230-5 and 230-6 can be gained by pulling out the end walls 1020-2 of the jacket 1020.

Also in the protection devices of Figures 7 to 10 (as well as in other protection devices like overpressure valves and underpressure valves) it is optionally possible to associate (in series) one or more CMR flame arrester elements with the flame arrester block 230, and/or to provide two (or, possibly, more) flame arrester blocks 230 arranged concentrically, coaxially (along the X axis), with one flame arrester block 230 of smaller external diameter D arranged inside another flame arrester block 230 of greater external diameter D.

Various modifications may be made to the embodiments of flame arrester block 230, 230- 1, 230-2, 230-3 and 230-4 described above.

For example, the spacers 315, rather than being separate parts distinct from the plates 305 and 310, may be formed in a single piece with the plates 305 and / or with the plate 310, providing a thickening of the material thereof, for example in correspondence of the holes 405.

The plates 305 and 310 and / or the spacers 315 may be made so as to define, once stacked, generically transverse interspaces, even if not strictly orthogonal to the X axis. In particular, the plates 305 do not necessarily have flat surface.

Furthermore, the annulus-shaped plates 305 may have different shapes, for example elliptical, square, rectangular, diamond-shaped, star-shaped, lobed. More generally, the plates 305 may be plates of any shape in plan view and having an outer perimeter and at least one opening in the inner region inside the perimeter.